Heretofore, assembly tasks involving insertion of rigid parts into subassemblies and products by machine or robot has been hindered by the physics of the contacting parts and uncertainties in the positions of the mating parts. For example, if the directions of the forces applied to mate the parts is inappropriate the parts will "jam" and will not move until the direction is corrected. If the initial positions of the parts are incorrect due to inaccuracy of the assembly machine or the dimensions of the parts themselves, the parts will wedge together and will not move unless the parts are separated and the assembly step is repeated. (The common experience of a stuck bureau drawer is a common example of wedging.) A wide class of assembly tasks is described by this simple "peg in hole" geometry and its variants.
Present methods employed to guarantee mechanized assembly include loose part tolerances, additions of generous chamfers, vibration of the pieces to improve the probability of the parts mating, active control of part positions or speeds by sophisticated force sensors and actuators, and the use of passive compliances.
Passive compliance apparatus, in particular, have enjoyed practical success because of the economies they realize in terms of assembly equipment hardware complexity and cost. Moreover, because of their manner of operation they reduce the need for incorporating part-mating facilitation features (e.g., generous tolerance and chamfer) into the object and the workplace in which the object is to be inserted, which features might promote ease of assembly at the expense of appearance, functionality or performance. For instance, the remote center of compliance (RCC) device disclosed in U.S. Pat. No. 4,098,001 is effective in assembling axisymmetric parts with chamfers and tolerances which range from very slight clearance to negative (i.e., interference) fits. RCC devices function by responding to part mating forces which arise during assembly with compensating motions that adjust the parts into better alignment. The essential mechanism is a set of linkages and/or springs which comply about a point in space nearly coincident with the tip of the part to be inserted or the rim of the hole that receives the part. In other mechanisms, the effect of compliance is relatively far from the initial part-to-product contact point. In this case, forces and moments due to contact usually make the parts move away from the ideal insertion centerline. An adjustable RCC proposed in U.S. Pat. No. 4,477,975 improves the RCC device of U.S. Pat. No. 4,098,001 by allowing adjustment of the position of the compliance center with respect to the part along or a short lateral distance off this centerline.
A disadvantage of presently available RCCs is that their arrangement of links and/or springs responds primarily to forces and torques exerted in imaginary planes within which lies the common central axis or centerline of the RCC. That is, they respond to insertion induced mating forces that are coplanar with the RCC centerline which can be best appreciated from a side, or an "elevation" view of the apparatus, but not to torsional, or twisting forces about the apparatus centerline, which are best conceptualized in a "plan" view of the device. In other words, from a plan view perspective, a typical RCC responds to forces arising at the part chamfer by resisting twisting motion which would be needed for correct insertion. Moreover, their response to forces and torques exerted in planes containing the RCC centerline is to produce compensating motions exclusively in those same planes. To illustrate this point, the RCC of U.S. Pat. No. 4,098,001 explicitly requires torque preventing means for preventing twisting about the RCC centerline of the operator member or part to be inserted into a workplace. Such an RCC is thus adequate for peg and hole geometries which are axisymmetric (circular), and for parts which are not circular in cross section (non-axisymmetric) but have no positioning error about their insertion axes (which are typically virtually coincidental with the RCC centerline). Since it remains highly difficult, if not impossible, to eliminate insertion axis positioning errors in respect to non-axisymmetric parts, known RCCs often fail to properly insert such parts into mating holes, e.g., square pegs into square holes.
An advantage exists, therefore, for an apparatus which is capable of inserting both axisymmetric and non-axisymmetric parts into correspondingly shaped holes under circumstances where positioning errors exist with respect to the insertion axes of such parts.